As he nurses his strong opinion-poll lead in the presidential election more than 13 months away, Governor George W Bush of Texas has demonstrated an enviable ability to dodge his opponents' best political blows.

Neither Mr Bush's youthful drug and drink problems, nor the revelation earlier this week that a family friend helped him to dodge the Vietnam era draft by securing him a plum posting at home in the Texas national guard, seems to have hampered his apparently relentless march towards the Republican party's presidential nomination next year.

The key tests for Mr Bush, most political observers now believe, will come as he begins to set out a detailed policy agenda - of which yesterday's defence speech at Charleston was a crucial element - and in the way he responds to the continuing attempts of his opponents and the investigative media to unearth the dirt on his youthful life in the privileged fast lane.

This week there are signs that one man may have begun to succeed where the teams of political professionals and the rest of the media have failed.

That man is the cartoonist Garry Trudeau, whose Doonesbury strip - published each day in the Guardian as well as in dozens of American newspapers - has exposed, well, not exactly the smoking gun from Mr Bush's past, but at least the smoking buttock.

It has been claimed for some months that when he was a Yale undergraduate in the 60s, Mr Bush joined, and later became president of, the college fraternity Delta Kappa Epsilon, whose initiation rites involved branding each newly enrolled member on the backside with a red-hot branding iron.

On Tuesday Doonesbury interrupted its normal cartoon format with a photograph, taken circa 67, of a flesh burn in the shape of the Greek letter Delta on an unidentified undergraduate Yale buttock. The photo, which shows the DKE brand, was taken by Martin Oppenheimer, now a Washington investment manager, who snapped it when he was working for the Yale Daily News and Mr Bush was president of the "Deke" fraternity.

The Bush press office is taking the exposure in good part. "I cannot confirm or deny that George W Bush was chairman of the Deke house who blackballed Garry Trudeau," a spokeswoman told the Post. Mr Bush, meanwhile, is saying nothing on the issue.

The spokeswoman's light-hearted treatment of the Doonesbury campaign cannot disguise the Bush camp's serious approach to handling stories about the governor's youth.

Although the US public's mood appears, in the aftermath of the Clinton impeachment, to be tolerant of the private problems of public figures, there is no doubt that Mr Bush's opponents think he is vulnerable to accusations of enjoying a privileged upbringing and a misspent youth which may ultimately damage his presidential hopes.

Only this week the Washington Post reported that the then speaker of the Texas legislature, Ben Barnes, intervened in 1967 after a request from Mr Bush's father, then a Texas congressman and later US president, to obtain a post for his son in a national guard fighter squadron at the height of the Vietnam war.

The question of how Mr Bush obtained such a post, which meant he would not be drafted for Vietnam, only two weeks after graduating from Yale, remains one of many potential embarrassments for him in the months ahead.